Illumination system of a microlithographic projection exposure apparatus

ABSTRACT

An illumination system of a microlithographic projection exposure apparatus includes a light source to produce projection light beam, and a first and a second diffractive optical element between the light source and a pupil plane of the illumination system. The diffractive effect produced by each diffractive optical element depends on the position of a light field that is irradiated by the projection light on the diffractive optical elements. A displacement mechanism changes the mutual spatial arrangement of the diffractive optical elements. In at least one of the mutual spatial arrangements, which can be obtained with the help of the displacement mechanism, the light field extends both over the first and the second diffractive optical element. This makes it possible to produce in a simple manner continuously variable illumination settings.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims benefit under 35 USC120 to, international application PCT/EP2010/007953, filed Dec. 28,2010, which is hereby incorporated by reference in its entirety.

FIELD

The disclosure generally relates to illumination systems forilluminating a mask in microlithographic exposure apparatus, and inparticular to such systems in which a diffractive optical element isused to define an irradiance distribution in a pupil plane. Thedisclosure also relates to a method of operating such illuminationsystems.

BACKGROUND

Microlithography (also called photolithography or simply lithography) isa technology for the fabrication of integrated circuits, liquid crystaldisplays and other microstructured devices. The process ofmicrolithography, in conjunction with the process of etching, is used topattern features in thin film stacks that have been formed on asubstrate, for example a silicon wafer. In general, at each layer of thefabrication, the wafer is first coated with a photoresist which is amaterial that is sensitive to radiation, such as deep ultraviolet (DUV)light. Next, the wafer with the photoresist on top is exposed toprojection light in a projection exposure apparatus. The apparatusprojects a mask containing a pattern onto the photoresist so that thelatter is only exposed at certain locations which are determined by themask pattern. After the exposure the photoresist is developed to producean image corresponding to the mask pattern. Then an etch processtransfers the pattern into the thin film stacks on the wafer. Finally,the photoresist is removed. Repetition of this process with differentmasks results in a multi-layered microstructured component.

A projection exposure apparatus typically includes an illuminationsystem for illuminating the mask, a mask stage for aligning the mask, aprojection objective and a wafer alignment stage for aligning the wafercoated with the photoresist. The illumination system illuminates a fieldon the mask that may have the shape of a rectangular or curved slit, forexample.

In current projection exposure apparatus a distinction can be madebetween two different types of apparatus. In one type each targetportion on the wafer is irradiated by exposing the entire mask patternonto the target portion in one go. Such an apparatus is commonlyreferred to as a wafer stepper. In the other type of apparatus, which iscommonly referred to as a step-and-scan apparatus or scanner, eachtarget portion is irradiated by progressively scanning the mask patternunder the projection beam along a scan direction while synchronouslymoving the substrate parallel or anti-parallel to this direction.

It is to be understood that the term “mask” (or reticle) is to beinterpreted broadly as a patterning mechanism. Commonly used maskscontain transmissive or reflective patterns and may be of the binary,alternating phase-shift, attenuated phase-shift or various hybrid masktype, for example. However, there are also active masks, e.g. masksrealized as a programmable mirror array. Also programmable LCD arraysmay be used as active masks.

As the technology for manufacturing microstructured devices advances,there are ever increasing demands also on the illumination system.Ideally, the illumination system illuminates each point of theilluminated field on the mask with projection light having a welldefined irradiance and angular distribution. The term angulardistribution describes how the total light energy of a light bundle,which converges towards a particular point in the mask plane, isdistributed among the various directions of the rays that constitute thelight bundle.

The angular distribution of the projection light impinging on the maskis usually adapted to the kind of pattern to be projected onto thephotoresist. For example, relatively large sized features may involve adifferent angular distribution than small sized features. The mostcommonly used angular distributions of projection light are referred toas conventional, annular, dipole and quadrupole illumination settings.These terms refer to the irradiance distribution in a pupil plane of theillumination system. With an annular illumination setting, for example,only an annular region is illuminated in the pupil plane. Thus there isonly a small range of angles present in the angular distribution of theprojection light, which means that all light rays impinge obliquely withsimilar angles onto the mask.

Different ways are known for modifying the angular irradiancedistribution of the projection light in the mask plane so as to achievethe desired illumination setting. In the simplest case a stop(diaphragm) including one or more apertures is positioned in a pupilplane of the illumination system. Since locations in a pupil planetranslate into angles in a Fourier related field plane such as the maskplane, the size, shape and location of the aperture(s) in the pupilplane are involved in determining the angular distributions in the maskplane. However, any change of the illumination setting involves areplacement of the stop. This can make it difficult to finely adjust theillumination setting, because this would typically involve a very largenumber of stops that have aperture(s) with slightly different sizes,shapes or locations. Furthermore, the use of stops inevitably results inlight losses and thus reduces the throughput of the apparatus.

Light losses caused by stops are avoided if diffractive optical elementsare used to produce a specific irradiance distribution in the pupilplane of the illumination system. The irradiance distribution can bemodified, at least to a certain extent, by adjustable optical elementssuch as zoom lenses or a pair of axicon elements that are arrangedbetween the diffractive optical element and the pupil plane.

Flexibility in producing different irradiance distributions in the pupilplane can be increased by using mirror arrays instead of the diffractiveoptical elements. For example, EP 1 262 836 A1 proposes the use of amirror array that is realized as a microelectromechanical system (MEMS)including more than 1000 microscopic mirrors. Each of the mirrors can betilted in two different planes perpendicular to each other. Thusradiation incident on such a mirror device can be reflected into(substantially) any desired direction of a hemisphere. A condenser lensarranged between the mirror array and the pupil plane translates thereflection angles produced by the mirrors into locations in the pupilplane. This prior art illumination system makes it possible toilluminate the pupil plane with a plurality of spots, wherein each spotis associated with one particular microscopic mirror and is freelymovable across the pupil plane by tilting this mirror.

Similar illumination systems are known from US 2006/0087634 A1, U.S.Pat. No. 7,061,582 B2, WO 2005/026843 A2 and WO 2010/006687.

However, in general, the use of mirror arrays is technologicallydemanding and involves sophisticated optical, mechanical andcomputational solutions.

A simpler approach to produce continuously variable irradiancedistributions in the pupil plane is the use of diffractive opticalelements having position dependent diffractive effects. Depending on theposition where the projection light impinges on the element, differentirradiance distributions are produced in the pupil plane. Usually theprojection light beam will be kept fixed and the diffractive opticalelement is displaced with the help of a displacement mechanism so as tochange the position where the projection light beam impinges on theelement. Diffractive optical elements of this kind are commerciallyavailable from Tessera Technologies, Inc., San Jose, USA.

However, the flexibility to produce different irradiance distributionsin the pupil plane can be quite restricted with such diffractive opticalelements. Generally, at most there are two available degrees of freedomthat can be used to modify this irradiance distribution, namely movingthe diffractive optical element along one direction and moving it alongan orthogonal direction. Displacing the diffractive optical elementalong the optical axis generally has very little effect on theirradiance distribution.

SUMMARY

Even with the additional flexibility provided by displaceable opticalelements such as zoom lenses and axicon elements, it remains desirableto provide increased flexibility to produce continuously variableirradiance distributions in the pupil plane.

The disclosure provides an illumination system which makes it possibleto produce continuously variable irradiance distributions in a pupilplane of the illumination system in a simple manner. The disclosure alsoprovides a method of operating an illumination system of amicrolithographic projection exposure apparatus which enables anoperator to produce continuously variable irradiance distributions in apupil plane of the illumination system in a simple manner.

In one aspect, the disclosure provides an illumination system of amicrolithographic projection exposure apparatus including a light sourcewhich is configured to produce a projection light beam, a pupil plane,an optical axis and first and a second diffractive optical elements. Thediffractive optical elements are arranged between the light source andthe pupil plane such that an irradiance distribution of projection lightin the pupil plane depends on diffractive effects produced by thediffractive optical elements. The diffractive effect produced by eachdiffractive optical element depends on the position of a light fieldthat is irradiated by the projection light beam on the diffractiveoptical elements. The illumination system further includes adisplacement mechanism which is configured to change the mutual spatialarrangement of the diffractive optical elements. In at least one of themutual spatial arrangements, which can be obtained with the help of thedisplacement mechanism, the light field extends both over the first andthe second diffractive optical element.

The disclosure is based on the concept that the range of possibleirradiance distributions can be considerably increased if the projectionlight beam impinges on not only one, but on two or more diffractiveoptical elements at a given instant. The irradiance distribution in thepupil plane will then depend on the mutual spatial arrangement of thediffractive optical elements. Since at least one degree of freedom isassociated with each diffractive optical element, the disclosureprovides at least two independent degrees of freedom to vary theirradiance distribution in the pupil plane.

This does not necessarily imply that the displacement mechanism iscapable of displacing each diffractive optical element independently. Ifthe illumination system includes a beam steering device that isconfigured to change the position of the light field if the diffractiveoptical elements are momentarily stationary, it suffices to configurethe displacement mechanism such that it is capable of displacing onlyone of the two diffractive optical elements relative to the other whichremains at a fixed position. Such a beam steering device may include anactuator that is configured to tilt, to displace or to deform an opticalelement, in particular a lens or a mirror.

In many cases it will be advantageous if the displacement mechanism is amotor driven mechanism which is configured to change the mutual spatialarrangement of the diffractive optical elements in response to an inputcommand from a control unit of the illumination system. The term “motor”should be understood broadly. It encompasses any kind of actuatingdevice using an external energy source and includes electric, pneumaticor piezoelectric motors, for example. With a motor driven mechanism theirradiance distribution in the pupil plane can be varied very quicklywithout a need to manually adjust the diffractive optical elements. Inprinciple, however, the displacement mechanism may also be a manuallydriven mechanism which is configured to change the mutual spatialarrangement of the diffractive optical elements when an operatormanually operates a lever or any other kind of operating element that ismechanically connected to the diffractive optical elements.

If the projection light beam propagates parallel to the optical axis ofthe illumination system, the displacement mechanism should be configuredto displace at least one diffractive optical element along adisplacement direction which is perpendicular or at least not parallelto the optical axis. Only then a displacement of the at least onediffractive optical element will have the desired effect that theposition of the light field that is irradiated by the projection lightbeam on the at least one diffractive optical element is changed.

Under such conditions the diffractive effects produced by the at leastone diffractive optical element should vary depending on the position ofthe light field along the displacement direction.

If the diffractive optical elements are substantially planar elements,they may extend in the same plane or in parallel planes.

In most embodiments the first and the second diffractive optical elementare arranged such that in at least one, preferably in all, mutualspatial arrangements, which can be obtained with the help of thedisplacement mechanism, projection light that impinges on the firstdiffractive optical element does not impinge on the second diffractiveoptical element. In other words, there will generally be projectionlight that impinges on the first diffractive optical element andprojection light that impinges on the second diffractive opticalelement, but no projection light that impinges on both diffractiveoptical elements. Then the combined irradiance distribution in the pupilplane will be a superposition of the irradiance distributions that areproduced individually by those portions of the first and the seconddiffractive optical element that lie within the light field.

If in at least one of the mutual spatial arrangements, which can beobtained with the help of the displacement mechanism, projection lightthat impinges on the first diffractive optical element also impinges onthe second diffractive optical element, the combined irradiancedistribution in the pupil plane will—for the overlapping portion of thetwo diffractive optical elements—not be a superposition, but aconvolution of the individual irradiance distributions produced by thefirst and the second diffractive optical element. By changing the mutualspatial arrangement of the first and the second optical element it ispossible to modify this convolution as desired.

In one embodiment the first and the second diffractive optical elementsare identical. The diffractive effect produced by each diffractiveoptical element varies, depending on the position of the light field,exclusively along one displacement direction. One of the diffractiveoptical elements is mounted in an orientation that is obtained byrotating the diffractive optical element by 180° around an axis that isparallel to the optical axis.

With two diffractive optical elements having such mirror symmetricaldiffractive effects there is only one degree of freedom, andconsequently the flexibility to produce different irradiancedistributions in the pupil plane is reduced. However, in such anembodiment small changes of the position of the light field along thedisplacement direction have very little effect on the irradiancedistribution in the pupil plane, because variations of the diffractiveeffect produced by one diffractive optical element are compensated bycounter effects produced by the other diffractive optical element.

This is important in those cases in which the position of the lightfield cannot be sufficiently stabilized. Oscillatory movements of thelight field can be caused by lasers that are used as light sources inthe illumination systems. The direction and also the divergence of thelight emitted by the laser are not perfectly stable, and over the longdistance of the beam delivery path (up to 20 meters) even very smallfluctuations result in significant shifts of the light field over thediffractive optical elements. With a mirror symmetrical arrangement ofthe diffractive optical elements, the sensitivity of the irradiancedistribution in the pupil plane against such fluctuations issignificantly reduced.

In order to suppress adverse effects caused by such fluctuations alsoalong the direction which is perpendicular to the displacementdirection, it is usually sufficient to design the diffractive opticalelements such that the light field has a height perpendicular to theoptical axis and to the displacement direction that is at least 5%,preferably at least 20%, smaller than the sum of the height of the firstand the second diffractive optical element.

In some embodiments the illumination system is configured such that thespatial arrangement of the diffractive optical elements relative to theprojection light beam can be changed along two orthogonal directionsthat do not include the optical axis. To change the spatial arrangementof the diffractive optical elements relative to the projection lightbeam either the diffractive optical elements themselves can be displacedwith the help of the displacement mechanism, or the projection lightbeam is moved, for example by changing its propagation direction using abeam steering device. In both cases it is possible to vary the areas ofthe portions on the diffractive optical elements over which the lightfield extends at a given instant. This, in turn, influences the ratio ofthe light energy which is distributed among the diffractive opticalelements.

For example, in at least one mutual spatial arrangement the light fieldextends over a first portion of the first diffractive optical elementand over a second portion of the second diffractive optical element,wherein the areas of the first and the second portion are different.Assuming that the projection light beam has a symmetric irradiancedistribution across its diameter, the amount of light which impinges onthe first and the second diffractive optical element will then bedifferent. Consequently also the irradiance distributions associatedwith each diffractive optical element will contain different amounts oflight energy.

The displacement mechanism may also be configured to displace thediffractive optical elements individually along two orthogonaldirections that do not include the optical axis.

In some embodiments the diffractive effect produced by the firstdiffractive optical element results in an irradiance distribution in thepupil plane having the shape of an annulus, wherein the width of theannulus depends on the position of the light field on the firstdiffractive optical element. The diffractive effect produced by thesecond diffractive optical element results in an irradiance distributionin the pupil plane including two poles, wherein the size of the polesdepends on the position of the light field on the second diffractiveoptical element. If the light field extends over the first and also thesecond diffractive optical element, the combined effect will be asuperposition of an annular and a dipole illumination setting.

There may be at least one additional optical element that is arrangedbetween the diffractive optical elements and the pupil plane. A furtherdisplacement mechanism may be provided that is configured to displacethe at least one optical element along an optical axis of theillumination system. The optical element may be formed by a lens or anaxicon element, for example. Then it is possible to modify theirradiance distribution in the pupil plane by additionally moving one ormore of the optical elements along the optical axis, as it is known assuch in the prior art.

The illumination system may also include a third diffractive opticalelement. Then there may be at least one mutual spatial arrangement inwhich the light field extends over the first, the second and also thethird diffractive optical element. The result in the pupil plane will bea superposition of the irradiance distributions that are produced by thefirst, the second and the third diffractive optical element.

In one aspect, the disclosure provides an illumination system of amicrolithographic projection exposure apparatus including a light sourcewhich is configured to produce a projection light beam, a pupil plane,an optical axis and first and a second diffractive optical elements. Thediffractive optical elements are arranged between the light source andthe pupil plane such that an irradiance distribution of projection lightin the pupil plane depends on diffractive effects produced by thediffractive optical elements. The diffractive effect produced by eachdiffractive optical element depends on the position where projectionlight impinges on the diffractive optical elements. The illuminationsystem further includes a displacement mechanism which is configured tochange the mutual spatial arrangement of the diffractive opticalelements. In at least one of the mutual spatial arrangements, which canbe obtained with the help of the displacement mechanism, projectionlight that has impinged on the first diffractive optical element alsoimpinges on the second diffractive optical element.

The illumination system according to this aspect of the disclosure isbased on the same general conception as outlined above. The range ofpossible irradiance distributions can be considerably increased if theprojection light beam impinges subsequently on two or more diffractiveoptical elements. The irradiance distribution in the pupil plane canthen be described as a convolution of the individual irradiancedistributions produced by the first and the second diffractive opticalelement. By changing the mutual spatial arrangement of the first and thesecond optical element it is possible to modify this convolution asdesired. Since at least one degree of freedom is associated with eachdiffractive optical element, the disclosure according to this aspectalso provides at least two independent degrees of freedom to vary theirradiance distribution in the pupil plane.

In one aspect, the disclosure provides a method which includes:

a) providing an illumination system including

-   -   i) a pupil plane,    -   ii) an optical axis,    -   iii) a first and a second diffractive optical element, wherein        -   the diffractive optical elements are arranged between a            light source and the pupil plane such that an irradiance            distribution of projection light in the pupil plane depends            on diffractive effects produced by the diffractive optical            elements, and wherein        -   the diffractive effect produced by each diffractive optical            element depends on the position of a light field that is            irradiated by a projection light beam on the respective            diffractive optical element;

b) producing the projection light beam;

c) changing the mutual spatial arrangement of the diffractive opticalelements;

wherein in at least one of the mutual spatial arrangements the lightfield extends both over the first and the second diffractive opticalelement.

The advantages of the method correspond to those noted above withrespect to the illumination system.

At least one diffractive optical element may be displaced along adisplacement direction which is perpendicular, or at least not parallel,to the optical axis.

The diffractive effect produced by the at least one diffractive opticalelement may vary, depending on the position of the light field, alongthe displacement direction.

The spatial arrangement of the diffractive optical elements relative tothe light field may be changed along two orthogonal directions that donot include the optical axis. This may be achieved by displacing thediffractive optical elements along two orthogonal directions that do notinclude the optical axis.

Then, in at least one mutual spatial arrangement, the light field mayextend over a first portion of the first diffractive optical element andover a second portion of the second diffractive optical element, whereinthe areas of the first and the second portion are different.

The light field may be moved by steering the projection light beam usinga beam steering device.

The illumination system may include a third diffractive optical element,and there may be at least one mutual spatial arrangement in which thelight field extends over the first, the second and the third diffractiveoptical element.

In one aspect, the disclosure provides a method which includes:

a) providing an illumination system including

-   -   i) a pupil plane,    -   ii) an optical axis,    -   iii) a first and a second diffractive optical element, wherein        -   the diffractive optical elements are arranged between a            light source and the pupil plane such that an irradiance            distribution of projection light in the pupil plane depends            on diffractive effects produced by the diffractive optical            elements, and wherein        -   the diffractive effect produced by each diffractive optical            element depends on the position of a light field that is            irradiated by a projection light beam on the respective            diffractive optical element;

b) producing a projection light beam;

c) changing the mutual spatial arrangement of the diffractive opticalelements;

wherein in at least one of the mutual spatial arrangements, which can beobtained with the help of the displacement mechanism, projection lightthat has impinged on the first diffractive optical element also impingeson the second diffractive optical element.

Definitions

The term “light” is used herein to denote any electromagnetic radiation,in particular visible light, UV, DUV, VUV and EUV light and X-rays.

The term “light ray” is used herein to denote light whose path ofpropagation can be described by a line.

The term “light bundle” is used herein to denote a plurality of lightrays that have a common origin in a field plane.

The term “light beam” is used herein to denote all the light that passesthrough a particular lens or another optical element.

The term “position” is used herein to denote the location of a referencepoint of a body or an immaterial object (such as light) in thethree-dimensional space. The position is usually indicated by a set ofthree Cartesian coordinates. The orientation and the position thereforefully describe the placement of a body in the three-dimensional space.

The term “direction” is used herein to denote the spatial orientation ofa straight line. A movement of an object along a specific direction thusimplies that the object is allowed to move in two opposite senses onthat line.

The term “surface” is used herein to denote any plane or curved surfacein the three-dimensional space. The surface may be part of a body or maybe completely separated therefrom, as it is usually the case with afield or a pupil plane.

The term “field plane” is used herein to denote a plane that isoptically conjugate to the mask plane.

The term “pupil plane” is used herein to denote a plane in whichmarginal rays passing through different points in the mask planeintersect. As usual in the art, the term “pupil plane” is also used ifit is in fact not a plane in the mathematical sense, but is slightlycurved so that, in the strict sense, it should be referred to as pupilsurface.

The term “optical raster element” is used herein to denote any opticalelement, for example a lens, a prism or a diffractive optical element,which is arranged, together with other identical or similar opticalraster elements so that each optical raster element is associated withone of a plurality of adjacent optical channels.

The term “optical integrator” is used herein to denote an optical systemthat increases the product NA·a, wherein NA is the numerical apertureand a is the area of the illuminated field.

The term “condenser” is used herein to denote an optical element or anoptical system that establishes (at least approximately) a Fourierrelationship between two planes, for example a field plane and a pupilplane.

The term “conjugated plane” is used herein to denote planes betweenwhich an imaging relationship is established. More information relatingto the concept of conjugate planes are described in an essay E. Delanoentitled: “First-order Design and the y, y Diagram”, Applied Optics,1963, vol. 2, no. 12, pages 1251-1256.

The term “spatial irradiance distribution” is used herein to denote howthe total irradiance varies over a real or imaginary surface on whichlight impinges. Usually the spatial irradiance distribution can bedescribed by a function I_(s)(x, y), with x, y being spatial coordinatesof a point on the surface. If applied to a field plane, the spatialirradiance distribution necessarily integrates the irradiances producedby a plurality of light bundles.

The term “angular irradiance distribution” is used herein to denote howthe irradiance of a light bundle varies depending on the angles of thelight rays that constitute the light bundle. Usually the angularirradiance distribution can be described by a function I_(a)(α, β), withα, β being angular coordinates describing the directions of the lightrays.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present may be more readilyunderstood with reference to the following detailed description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic perspective view of a projection exposureapparatus in accordance with one embodiment of the present disclosure;

FIG. 2 is a meridional section through an illumination system of theapparatus shown in FIG. 1 according to a first embodiment;

FIG. 3 shows in its upper portion a top view of a first diffractiveoptical element contained in the illumination system shown in FIG. 2 andin its lower portion three irradiance distributions that are produced inthe far field by the first diffractive optical element if irradiatedwith a light field at different positions;

FIG. 4 shows in its upper portion a top view of a second diffractiveoptical element contained in the illumination system shown in FIG. 2 andin its lower portion three irradiance distributions that are produced inthe far field by the second diffractive optical element if irradiatedwith a light field at different positions;

FIG. 5 is a top view on the first and the second diffractive opticalelement if viewed along the optical axis in a specific mutualarrangement;

FIG. 6 shows a plurality of different combined irradiance distributionsthat can be obtained with different mutual spatial arrangements of thefirst and the second diffractive optical element;

FIG. 7 is a meridional section through an illumination system of theapparatus shown in FIG. 1 according to a second embodiment in which thediffractive optical elements can be displaced along two orthogonaldirections;

FIGS. 8a to 8c are top views on the first and the second diffractiveoptical element if viewed along the optical axis in a specific mutualarrangement, but with different relative positions with respect to theprojection light beam;

FIG. 9 is a meridional section through an illumination system of theapparatus shown in FIG. 1 according to a third embodiment in which thespatial arrangement of the diffractive optical elements relative to theprojection light beam can be varied by a beam steering device;

FIG. 10 is a top view on three diffractive optical elements if viewedalong the optical axis in a specific mutual arrangement according to afourth embodiment;

FIG. 11 is a top view on two overlapping diffractive optical elementsaccording to a fifth embodiment if viewed along the optical axis;

FIGS. 12a to 12c are top views on two diffractive optical elementsaccording to a sixth embodiment in which the diffractive opticalelements have diffractive effects with a mirror symmetrical positiondependence; and

FIG. 13 is a flow diagram that illustrates important method steps.

DESCRIPTION OF PREFERRED EMBODIMENTS I. General Construction ofProjection Exposure Apparatus

FIG. 1 is a perspective and highly simplified view of a projectionexposure apparatus 10 in accordance with the present disclosure. Theapparatus 10 includes an illumination system 12 which produces aprojection light beam (not shown). The latter illuminates a field 14 ona mask 16 containing a pattern 18 formed by a plurality of smallfeatures 19 that are schematically indicated in FIG. 1 as thin lines. Inthis embodiment the illuminated field 14 has the shape of a ringsegment. However, other shapes of the illuminated field 14, for examplerectangles, are contemplated as well.

A projection objective 20 images the pattern 18 within the illuminatedfield 14 onto a light sensitive layer 22, for example a photoresist,which is supported by a substrate 24. The substrate 24, which may beformed by a silicon wafer, is arranged on a wafer stage (not shown) suchthat a top surface of the light sensitive layer 22 is precisely locatedin an image plane of the projection objective 20. The mask 16 ispositioned via a mask stage (not shown) in an object plane of theprojection objective 20. The projection objective 20 has a magnificationwith an absolute value of less than one. Accordingly, a minified image18′ of the pattern 18 within the illuminated field 14 is projected ontothe light sensitive layer 22.

In this embodiment the design of the projection objective 20 theilluminated field 14 is positioned off the optical axis 26 of theprojection objective 20. With other types of projection objectives, theilluminated field 14 may be centered on the optical axis 26.

During the projection the mask 16 and the substrate 24 move along a scandirection which corresponds to the Y direction indicated in FIG. 1. Theilluminated field 14 then scans over the mask 16 so that patterned areaslarger than the illuminated field 14 can be continuously imaged. Theratio between the velocities of the substrate 24 and the mask 16 isequal to the magnification of the projection objective 20. If theprojection objective 20 inverts the image (has a magnification with avalue of less than zero), the mask 16 and the substrate 24 move inopposite senses, as this is indicated in FIG. 1 by arrows A1 and A2.However, the present disclosure may also be used in stepper tools inwhich the mask 16 and the substrate 24 do not move during projection ofthe mask.

II. General Construction of Illumination System

FIG. 2 is a meridional section through the illumination system 12 shownin FIG. 1. For the sake of clarity, the illustration of FIG. 2 isconsiderably simplified and not to scale. This particularly implies thatdifferent optical units are represented by one or very few opticalelements only. In reality, these units may include significantly morelenses and other optical elements.

The illumination system 12 includes a housing 28 and a light source 30that is, in the embodiment shown, realized as an excimer laser. Thelight source 30 emits projection light having a wavelength of about 193nm. Other types of light sources 30 and other wavelengths, for example248 nm or 157 nm, are also contemplated.

In the embodiment shown, the projection light emitted by the lightsource 30 enters a beam expansion unit 32 which outputs an expanded andalmost collimated projection light beam 34. In order to increase thediameter of the projection light beam, the beam expansion unit 32 mayinclude several lenses or may be realized as a mirror arrangement, forexample.

After having been deflected at a first planar beam path folding mirror36 and a second planar beam path folding mirror 38, the projection lightbeam 34 enters a pupil defining unit 40 that is used to produce variablespatial irradiance distributions in a subsequent pupil plane. To thisend the pupil defining unit 40 includes a first diffractive opticalelement 42 and a second diffractive optical element 44.

As can best be seen in the enlarged cut-out 46, each of the diffractiveoptical elements 42, 44 includes a plurality of minute diffractivestructures 48 that are formed on a common planar substrate 50. Thediffractive optical elements 42, 44 may be realized as computergenerated holograms (CGH), as it is known in the art as such. Theoptical properties of the first and the second diffractive opticalelement 42, 44 will be explained further below with reference to FIGS. 3and 4.

The diffractive optical elements 42, 44 extend in planes that areorthogonal to an optical axis OA of the illumination system 12. In thisembodiment the two planes are slightly displaced along the optical axisOA, but it may also be envisaged to arrange the diffractive opticalelements 42, 44 in a common plane. As it will become clear from FIG. 5,which shows the two diffractive optical elements 42, 44 if viewed alongthe Z direction to which the optical axis OA is parallel, the twodiffractive optical elements 42, 44 are arranged in such a manner thatthey do not overlap, but are immediately contiguous to each other alongthe Y direction.

The pupil defining unit 40 further includes a displacement mechanism 52that is configured to change the mutual spatial arrangement of thediffractive optical elements 42, 44 by displacing the diffractiveoptical elements 42, 44 individually along the X direction. To this endthe displacement mechanism 52 includes a first driver 54 that isconfigured to displace the first diffractive optical element 42 alongthe X direction, as it is indicated by a double arrow A3. In thisembodiment the first driver 54 includes a servo motor 56 which isconnected to the first diffractive optical element 42 by a toothedgearing 58. The servo motor 56 is controlled by a control unit 60 whichis connected to an overall system control 62.

The displacement mechanism 52 further includes a second driver 64 thatis associated with the second diffractive optical element 44 in asimilar manner. The second driver 64 is configured to displace thesecond diffractive optical element 44 along the X direction, as it isindicated by a double arrow A4. Similar to the first driver 54, thesecond driver 64 includes a servo motor 66 that is connected to thesecond diffractive optical element 44 by a toothed gearing 68. Also theservo motor 66 of the second displacement mechanism 64 is connected tothe control unit 60.

In this way the mutual spatial arrangement of the first and the seconddiffractive optical element 42, 44 can be changed in response to aninput command from the control unit 60.

The pupil defining unit 40 also includes a zoom collimator lens 70,which can be displaced along the optical axis OA with the help of afirst actuator 71 (see double arrow A5), and a first and a second axiconelement 72, 74 having complementary conical surfaces. The distancebetween the axicon elements 72, 74 along the optical axis OA can bechanged with the help of a second actuator 76, as it is indicated by adouble arrow A6. In this embodiment the second actuator 76 is coupledonly to the second axicon element 74; other configurations to change thedistance between the two axicon elements 72, 74 are also feasible. Theaxicon elements 72, 74 have the effect that an irradiance distributionat the entrance surface of the first axicon element 72 is radiallyshifted outwardly. The amount of radial shift depends on the distancebetween the first axicon element 72 and the second axicon element 74.

Light having passed the axicon elements 72, 74 impinges on an opticalintegrator 78 which includes, in the embodiment shown, two arrays 80, 82of optical raster elements. Each optical raster element is formed bycrossing two cylindrical lenses, as it is known in the art as such. Theoptical raster elements may also be formed by rotationally symmetriclenses having a rectangular borderline, for example. The opticalintegrator 78 produces in a pupil plane 84 a plurality of secondarylight sources. Each secondary light source is associated with an opticalchannel which is defined by two optical raster elements of the arrays80, 82 having the same X and Y coordinates.

A condenser 86 transforms the angular light distribution produced by thesecondary light sources into a spatial irradiance distribution at asubsequent intermediate field plane 88. Since all secondary lightsources produce substantially the same angular irradiance distribution,also the spatial irradiance distributions in the intermediate fieldplane 88 are very similar. The superpositions of these irradiancedistributions results in a very homogenous illumination of a field inthe intermediate field plane 88.

The intermediate field plane 88 is imaged, together with a field stop90, by a field stop objective 92 onto a mask plane 94 in which the mask16 is arranged. The field 14 illuminated on the mask 16 is thus an imageof the field which is illuminated in the intermediate field plane 88 bythe plurality of secondary light sources and which is masked by thefield stop 90.

III. Optical Properties of Diffractive Optical Elements

In the following the optical properties of the first and the seconddiffractive optical element 42, 44 will be explained in more detail withreference to FIGS. 3 and 4.

FIG. 3 shows in its upper portion a top view of the first diffractiveoptical element 42. The diffractive structures 48, which are not shownfor the sake of simplicity, of the first diffractive optical element 42are designed such that the diffractive effect produced by the firstdiffractive optical element 42 depends on the position of a light fieldthat is irradiated by the projection light beam 34 on the firstdiffractive optical element 42.

In FIG. 3 a first, a second and a third position of such a light fieldare shown and denoted by 96 a, 96 b and 96 c, respectively. The threepositions 96 a, 96 b, 96 c differ from one another only with respect totheir location along the X direction.

If the projection light beam 34 irradiates a light field at the firstposition 96 a on the first diffractive optical element 42, it willproduce an angular light distribution which corresponds in the far field(or after Fourier transformation by the zoom collimator lens 70, whichis equivalent) to a first spatial irradiance distribution which isdenoted in FIG. 3 by 98 a. In this first spatial irradiance distribution98 a only two small poles P1, P2 are illuminated that are spaced apartalong the X direction. Each pole P1, P2 has the shape of a segment of aring having an outer radius r_(po) and an inner radius r_(pi). Theangular extension of the ring segment, which will be referred to in thefollowing as pole width angle α, is identical for both poles P1, P2.

If the projection light beam 34 produces a light field at the secondposition 96 b on the first diffractive optical element 42, a similarspatial irradiance distribution will be produced in the far field, butwith poles P1, P2 having a larger pole width angle α. In the thirdposition 96 c of the light field the pole width angle α has its maximumvalue.

It is to be understood that in all intermediate positions of the lightfield similar poles P1, P2 will be produced, but with pole width anglesα having values which are between those that are indicated in FIG. 3 forthe three positions 96 a, 96 b and 96 c.

In this embodiment the different positions 96 a, 96 b, 96 c illustratedin FIG. 3 are not produced by moving the projection light beam 34 overthe fixed first diffractive optical element 42, but by displacing, withthe help of the first driver 54, the first diffractive optical element42 relative to the fixed projection light beam 34.

FIG. 4 shows, in a similar representation as FIG. 3, different spatialirradiance distributions 103 a, 103 b, 103 c that are produced in thefar field, if a light field is irradiated on the second diffractiveoptical element 44 at different X positions 102 a, 102 b, 102 c. In afirst position 102 a of the light field an annulus 104 having an outerradius r_(ao) and an inner radius r_(ai) is irradiated. If the lightfield is moved along the X direction (position 102 a), the outer radiusr_(ao) of the annulus 104 continuously increases until it reaches itsmaximum value shown for the third position 102 c.

In the following the function of the illumination system 12 will beexplained with reference to FIGS. 5 and 6.

IV. Function

FIG. 5 is a top view on the first and the second diffractive opticalelement 42, 44 if viewed along the Z direction which is parallel to theoptical axis OA. The diffractive optical elements 42, 44 are guided inguide rails 104, 106, 108 so that they can be individually displacedwith the help of the first and the second driver 54, 64 (not shown inFIG. 5) along the X direction.

In each diffractive optical element 42, 44 the far field irradiancedistributions are schematically indicated which will be produced iflight impinges on the respective X position of the diffractive opticalelements 42, 44. The two diffractive optical elements 42, 44 arearranged in the beam path of the projection light beam 34 such that thelatter irradiates on the diffractive optical elements 42, 44 a lightfield 110 which extends equally to both diffractive optical elements 42,44. In other words, one half of the light beam 34 (if one disregards thesmall light losses produced by the middle guiding rail 106) isdiffracted by the first diffractive optical element 42, and the otherhalf of the light beam 34 is diffracted by the second diffractiveoptical element 44.

In the specific mutual spatial arrangement of the first and the seconddiffractive optical element 42, 44 which is shown in FIG. 5, the firstdiffractive optical element 42 produces in the far field two light polesP1, P2 having a medium sized pole width angle α. The second diffractiveoptical element 44 produces in the far field a thin annulus 104 having asmall outer radius r_(ao). Since both far field irradiance distributionssimply superimpose, a combined irradiance distribution 112 is obtainedwhich is a combination of the thin annulus 104 with the two medium sizedpoles P1, P2.

By changing the mutual spatial arrangement of the first and the seconddiffractive optical element 42, 44, the thickness of the annulus 104 andthe pole width angle α can be independently varied. More specifically,if the first diffractive optical element 42 is moved along the Xdirection with the help of the first driver 54, as it is indicated bythe arrow A3, the pole width angle α continuously changes. If the seconddiffractive optical element 44 is moved along the X direction with thehelp of the second driver 64, as it is indicated by the arrow A4, theouter radius of the annulus 104 continuously changes.

This is illustrated also in FIG. 6 which shows a plurality of differentcombined irradiance distributions 112 that can be obtained withdifferent mutual spatial arrangements of the first and the seconddiffractive optical element 42, 44. In the first three rows of FIG. 6 ithas been assumed that the first diffractive optical element 42 shown inFIG. 5 has been moved to the right such that the light field 110 extendsover a portion of the first diffractive optical element 42 whichproduces in the far field poles P1, P2 having the smallest pole widthangle α. This spatial irradiance distribution is combined with differentannuli 104 that are produced in the far field by moving the seconddiffractive optical element 44 shown in FIG. 5 to the left. Then thegeometry and size of the poles P1, P2 in the combined irradiancedistribution 112 is kept fixed, while the outer radius r_(ao) of theannulus 104 continuously increases.

Depending on the mutual spatial arrangement of the first and the seconddiffractive optical element 42, 44 any arbitrary combination of polewidth angle α and outer radius r_(ao) of the annulus 104 can beobtained.

Each combined irradiance distribution 112 that can be obtained using thetwo diffractive optical elements 42, 44 can be further varied with thehelp of the zoom collimator lens 64 and the axicon elements 72, 74. Ifthe zoom collimator lens 64 is displaced along the optical axis OA, thiswill have the effect of magnifying or minifying the combined irradiancedistribution 112 that is produced by the diffractive optical elements42, 44 in their instant spatial arrangement. In other words, theirradiance distribution is scaled up or scaled down by a constantfactor. This entails, for example, that if the outer radius r_(ao) ofthe annulus 104 is increased by a factor of x, also its inner radiusr_(ai) is increased by a factor x.

With the help of the axicon elements 72, 74 the annulus 104 and thepoles P1, P2 can be moved radially without changing their radial size.This entails, for example, that if the outer radius r_(ao) of theannulus 104 is increased by a factor of x, its inner radius r_(ai)becomes r_(ai)+r_(ao)(x−1).

The illumination system 12 thus provides 4 degrees of freedom, namelythe X positions of the first and the second diffractive optical element42, 44, the Z position of the zoom collimator lens 64 and the Z positionof the second axicon element 74, to adjust the irradiance distributionin the pupil plane 84 so as to image the mask 16 in the best possibleway on the light sensitive surface 22.

V. Second Embodiment

FIG. 7 is a meridional section through an illumination system 12according to a second embodiment in a representation similar to FIG. 2.

The illumination system 12 shown in FIG. 7 differs from the illuminationsystem shown in FIG. 2 mainly in that the first and the second driver54, 64 are configured to displace the diffractive optical elements 42,44 not only along the X direction, but also along the Y direction. The Xand the Y directions are orthogonal to each other but do not include theoptical axis OA.

To this end the drivers 54, 64 include additional servo motors 114, 116that are capable of displacing the first and the second diffractiveoptical element 42, 44, together with the servo motors 56, 66 and thetoothed gearings 58, 68, along the Y direction.

The effect of being able to displace the first and the seconddiffractive optical element 42, 44 also along the Y direction will beexplained in the following with reference to FIGS. 8a, 8b and 8 c.

FIG. 8a is substantially identical to FIG. 5. The first and the seconddiffractive optical element 42, 44 are arranged relative to the lightfield 110 such that the portions irradiated on the first and the seconddiffractive optical element 42, 44 have at least substantially the samearea. Consequently the same amount of light is directed towards thepoles P1, P2 as to the annulus 104 in the combined irradiancedistribution 112.

In FIG. 8b it has been assumed that the two diffractive optical elements42, 44 have been displaced downward, as it is indicated by arrow 118,i.e. along the Y direction. Consequently, the arrangement of the firstand the second diffractive optical element 42, 44 relative to the lightfield 110 is changed so that the portions irradiated on the first andthe second diffractive optical element 42, 44 by the projection lightbeam 34 now have different areas. As a result more than one half of theavailable light is directed by the first diffractive optical element 42towards the poles P1, P2, and less than one half of the available lightis directed by the second diffractive optical element 44 towards theannulus 104. In other words, light energy is shifted from the annulus104 to the poles P1, P2 in the combined irradiance distribution 112.

If the two diffractive optical elements 42, 44 are commonly movedupwards along the Y direction, as it is indicated by an arrow 120 inFIG. 8c , most projection light will be directed towards the annulus104, and the poles P1, P2 are only weakly irradiated in the pupil plane84.

By changing the Y position of the diffractive optical elements 42, 44 itis thus possible to continuously vary the light energy ratio of theavailable light that is directed towards the poles P1, P2 and the lightthat is directed towards the annulus 104.

The ability to change the light energy ratio between the annulus 104 andthe poles P1, P2 by moving the diffractive optical elements 42, 44 alongthe Y direction is particularly advantageous if not only the position,but also the areas of the far field irradiance distributions produced byeach diffractive optical element 42, 44 are changed with movements ofthe diffractive optical elements 42, 44 along the X direction. Forexample, if the size of the poles P1, P2 shall be increased byincreasing the poles width angle α, it may be desirable to keep theirradiance at each point in the poles P1, P2 constant. Then light energycan be transferred from the annulus 104 to the poles P1, P2 to such anextent that the irradiance in the poles P1, P2 remains constantirrespective of the pole width angle α.

VI. Further Alternative Embodiments

a) Changing Beam Direction

FIG. 9 is a meridional section through an illumination system 12according to a third embodiment in which it is also possible to changethe spatial arrangement of the diffractive optical element 42, 44relative to the light field 110 along the X and Y direction. However, inthis embodiment the first and the second drivers 54, 64 are, similar tothe embodiment shown in FIG. 2, only capable to displace the diffractiveoptical elements 42, 44 along the X direction so as to change theirmutual spatial arrangement. The change of the arrangement of thediffractive optical elements 42, 44 relative to the light field 110along the Y direction is produced in this embodiment by a beam steeringdevice including an actuator 122. The latter is capable of tilting thesecond beam folding mirror 38 such that the projection light beam 34,and thus the light field 110 irradiated by the light beam 34 on thediffractive optical elements 42, 44, moves along the Y direction up anddown as desired. Then the sizes of the poles P1, P2 and the annulus 104are determined by the mutual spatial arrangement of the first and thesecond diffractive optical element 42, 44 along the X direction. Thelight energy ratio is determined by moving the light field 110 up anddown with the help of the actuator 122.

This concept of aiming the light beam 34 to a desired position on thediffractive optical elements 42, 44 using the actuator 122 helps to keepthe mechanical layout of the first and the second driver 54, 64 simple.

Since tilting the second beam folding mirror 38 inevitably changes thedirection of projection light beam 34 impinging on the diffractiveoptical elements 42, 44, it may be preferred to configure the actuator122 such that it is capable of displacing the second beam folding mirror38 along the X direction. If the diffractive optical elements 42, 44 andthe drives 54, 64 are mounted in a orientation which is obtained fromthe arrangement shown in FIG. 9 by rotating these components by 90°about optical axis OA, the same effect is achieved as in the embodimentshown in FIG. 9, but the projection light beam 34 always impinges underthe same angle on the diffractive optical elements 42, 44.

b) Three Diffractive Optical Elements

FIG. 10 shows, in a representation similar to FIG. 5, a fourthembodiment in which not only two, but three different diffractiveoptical elements 42, 44, 45 are arranged in one plane or in parallelplanes so that they do not overlap.

In this embodiment the first and the second diffractive optical element42, 44 have the same optical properties as it has been explained abovewith reference to FIGS. 3 and 4. The third diffractive optical element45 produces in the far field, depending on the position where the lightbeam 34 impinges, the same arrangement of poles P1, P2 as the firstdiffractive optical element 42, but rotated by 90°. With the threediffractive optical elements 42, 44, 45 it is then possible to producecombined irradiance distributions 112 with four poles P1, P2, P3, P4 andan outer annulus 104. However, the pole width angle α of the poles P1,P2 arranged along the X direction and of the poles P3, P4 arranged alongthe Y direction can be independently varied by displacing the first andthe third diffractive optical element 42, 45 along the X direction.

If additionally the arrangement of the diffractive optical elements 42,44, 45 relative to the light field 110 is changed along the Y direction,the energy ratio between the poles P1, P2 extending along the Xdirection and the poles P3, P4 extending along the Y direction changes.

c) Overlapping Diffractive Optical Elements

FIG. 11 is a schematic top view of a first and a second diffractiveoptical element 142, 144 according to a fifth embodiment. Unlike theembodiments described above, the diffractive optical elements 142, 144are arranged in different parallel planes so that they at least partlyoverlap. Then the projection light beam 34 passes subsequently throughthe first and then through the second diffractive optical element 142,144. The combined irradiance distribution 112 may in this case bedescribed as a convolution of the far field spatial irradiancedistributions produced by the first and by the second diffractiveoptical element 142, 144.

In the specific configuration shown in FIG. 11 it is assumed that thefirst diffractive optical element 142 produces a plurality of small dots146 whose number and/or positions depend on the X position of the lightfield 110 on the first diffractive optical element 142. The seconddiffractive optical element 144 is assumed to produce in the far field asingle central spot 148 having a circular boundary line. The diameter ofthe spot increases depending on the X position where light diffracted bythe first diffractive optical element 142 impinges on the seconddiffractive optical element 144.

By convoluting the two spatial irradiance distributions it is possibleto produce varying patterns of spots or poles 150 in the pupil plane 84.The pole pattern is determined by the X position of the firstdiffractive optical element 142, and the pole diameter is determined bythe X position of the second diffractive optical element 144.

d) Beam Oscillation Compensation

FIG. 12a is a top view, similar to the representation shown in FIG. 5,on two diffractive optical elements 242, 244 according to a sixthembodiment.

This embodiment differs from the first embodiment shown in FIGS. 2 to 6mainly in that the second diffractive optical element 244 does notproduce different annuli in the far field, but the same poles as thefirst diffractive optical element 242. However the directionaldependence is mirror symmetrically reversed. More specifically, if theposition of the light field 110 shown in FIG. 12a moves to the right,the size of the poles P1′, P2′ produced by the first diffractive opticalelement 242 increases, and the size of the poles P1″, P2″ produced bythe second diffractive optical element 244 decreases. This may simply beaccomplished by producing two identical diffractive optical elements,but mounting one of them after rotating it by 180° around an axis thatis parallel to the optical axis OA.

With such a configuration of the first and the second diffractiveoptical element 242, 244 the combined irradiance distribution 112 willbe at least approximately independent of the position of the light field110 of the first and the second diffractive optical element 242, 244.This is illustrated in FIG. 12b which shows how the light field 110irradiates the diffractive optical elements 242, 244 at an X positionshifted to the left. The first diffractive optical element 242 thenproduces smaller poles P1′, P2′, but this is compensated by the seconddiffractive optical element 244 which produces larger poles P1″, P2″.The poles P1, P2 of the combined far field irradiance distribution 112then have a size which is halfway in between the size of the poles P1′,P2′ and the size of the poles P1″, P2″.

As it is shown in FIG. 12c , also the position of the light field 110along the Y direction does not affect the combined irradiancedistribution 112. This is because both diffractive optical elements 242,244 direct light to the same poles, and thus both poles P1, P2 in thecombined irradiance distribution 112 receive the same amount of lightirrespective of the Y position of the light field 110 with respect tothe arrangement of the diffractive optical elements 242, 244.

Such an independence of the combined irradiance distribution 112 of theposition of the light field 110 on the diffractive optical elements 242,244 is advantageous in those cases in which it is difficult to spatiallystabilize the projection light beam 34 when it impinges on the pupildefining unit 40. Such undesired time dependent variation of the beamposition may be a result of certain drift effects in the light source 30which are greatly amplified by the long distance between the lightsource 30 and the pupil defining unit 40 (usually several or even up to20 meters). Then the proposed arrangement of identical diffractiveoptical elements 242, 244 ensures that the combined irradiancedistribution 112 in the pupil plane 84 is not significantly affected bysuch undesired oscillations of the light field 110.

Nevertheless it is possible to modify the pole width angle α by changingthe mutual spatial arrangement of the first and the second diffractiveoptical element 242, 244 with the help of the first and the seconddriver 54, 64.

VII. Method Steps

In the following important method steps of the disclosure will besummarized with reference to the flow diagram shown in FIG. 13.

In a first step S1 an illumination system is provided that includes afirst and the second diffractive optical element. In a second step S2 aprojection light beam is produced. In a third step S3 the mutual spatialarrangement of the diffractive optical elements is changed.

1.-20. (canceled)
 21. An illumination system, comprising: a light sourceconfigured to produce projection light; a first diffractive opticalelement; a second diffractive optical element; and a displacementmechanism configured to change a relative spatial arrangement of thefirst and second diffractive optical elements, wherein: the first andsecond diffractive optical elements are between the light source and apupil plane of the illumination system so that, during use of theillumination system, an irradiance distribution of the projection lightin the pupil plane depends on diffractive effects produced by both thefirst and second diffractive optical elements; for each of the first andsecond diffractive optical elements, the diffractive effect produced bythe diffractive optical element during use of the illumination systemcontinuously varies when a position of a light field of the projectionlight on an effective region of the diffractive optical element changescontinuously; in at least one relative spatial arrangement of the firstand second diffractive optical elements obtainable via the displacementmechanism, projection light that has impinged on the first diffractiveoptical element during use of the illumination system also impinges onthe second diffractive optical element; and the illumination system is amicrolithographic illumination system.
 22. The illumination system ofclaim 21, wherein: the first diffractive optical element is in a firstplane; the second diffractive optical element is in a second planedifferent from the first plane; the first plane is parallel to thesecond plane; and the first and second diffractive optical elements atleast partly overlap.
 23. The illumination system of claim 21, wherein:the displacement mechanism is configured to displace at least onediffractive element along a direction which is not parallel to anoptical axis of the illumination system; and the at least onediffractive element is selected from the group consisting of the firstdiffractive element and the second diffractive optical element.
 24. Theillumination system of claim 21, wherein, during use of the illuminationsystem, the diffractive effect produced by the at least one diffractiveoptical element continuously varies, depending on a position of thelight field, along the displacement direction.
 25. The illuminationsystem of claim 24, wherein, during use of the illumination system, thediffractive effect produced by the at least one diffractive opticalelement continuously varies, depending on a position of the light field,exclusively along the displacement direction.
 26. The illuminationsystem of claim 21, further comprising a beam steering device configuredto change a position of the light field.
 27. The illumination system ofclaim 21, wherein: the first diffractive optical element is configuredto produce a plurality of spots in a far field during use of theillumination system; and the second diffractive optical element isconfigured to produce a single spot in the far field during use of theillumination system.
 28. The illumination system of claim 27, wherein:during use of the illumination system, at least one parameter depends onthe position of the light field on the first diffractive opticalelement; and the at least one parameter is selected from the groupconsisting of a number of spots produced by the first diffractiveoptical element and positions of the spots produced by the firstdiffractive optical element.
 29. The illumination system of claim 28,wherein, during use of the illumination system, a diameter of the spotproduced by the second diffractive optical element depends on theposition of the light field on the second diffractive optical element.30. The illumination system of claim 27, wherein, during use of theillumination system, a diameter of the spot produced by the seconddiffractive optical element depends on the position of the light fieldon the second diffractive optical element.
 31. An apparatus, comprising:an illumination system according to claim 21; and a projectionobjective, wherein the apparatus is a microlithographic projectionexposure apparatus.
 32. A method of operating a microlithographicprojection exposure apparatus comprising an illumination system and aprojection objective, the method comprising: using the illuminationsystem to illuminate a mask; and using the projection objective toproject an image of at least a portion of the illuminated mask onto aphotoresist, wherein the illumination system is an illumination systemaccording to claim
 21. 33. An illumination system, comprising: a firstdiffractive optical element; a second diffractive optical element; and adisplacement mechanism configured to change a relative spatialarrangement of the first and second diffractive optical elements,wherein during use of the illumination system: the first and seconddiffractive optical elements are between the light source and a pupilplane of the illumination system so that an irradiance distribution oflight in the pupil plane depends on diffractive effects produced by boththe first and second diffractive optical elements; in at least onerelative spatial arrangement of the first and second diffractive opticalelements obtainable via the displacement mechanism, projection lightthat has impinged on the first diffractive optical element also impingeson the second diffractive optical element; the first diffractive opticalelement is configured to produce a plurality of spots in a far field; atleast one parameter depends on the position of the light field on thefirst diffractive optical element; and the at least one parameter isselected from the group consisting of a number of spots produced by thefirst diffractive optical element and positions of the spots produced bythe first diffractive optical element; at least one of a number andpositions of the spots produced by the first diffractive optical elementdepends on the position of the light field on the first diffractiveoptical element; the second diffractive optical element is configured toproduce a single spot in the far field; and a diameter of the spotproduced by the second diffractive optical element depends on theposition of the light field on the second diffractive optical element;and wherein the illumination system is a microlithographic illuminationsystem.
 34. The illumination system of claim 33, wherein: the firstdiffractive optical element is in a first plane; the second diffractiveoptical element is in a second plane different from the first plane; thefirst plane is parallel to the second plane; and the first and seconddiffractive optical elements at least partly overlap.
 35. Theillumination system of claim 33, wherein: the displacement mechanism isconfigured to displace at least one diffractive element along adirection which is not parallel to an optical axis of the illuminationsystem; and the at least one diffractive element is selected from thegroup consisting of the first diffractive element and the seconddiffractive optical element.
 36. The illumination system of claim 33,wherein, during use of the illumination system, the diffractive effectproduced by the at least one diffractive optical element continuouslyvaries, depending on the position of the light field, along thedisplacement direction.
 37. The illumination system of claim 36,wherein, during use of the illumination system, the diffractive effectproduced by the at least one diffractive optical element continuouslyvaries, depending on the position of the light field, exclusively alongthe displacement direction.
 38. The illumination system of claim 33,further comprising a beam steering device configured to change aposition of the light field.
 39. An apparatus, comprising: anillumination system according to claim 33; and a projection objective,wherein the apparatus is a microlithographic projection exposureapparatus.
 40. A method of operating a microlithographic projectionexposure apparatus comprising an illumination system and a projectionobjective, the method comprising: using the illumination system toilluminate a mask; and using the projection objective to project animage of at least a portion of the illuminated mask onto a photoresist,wherein the illumination system is an illumination system according toclaim 33.